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What are the basics of using objects in Delphi?
Delphi - The Basics
In this short tutorial, I'll talk about one of the most common practices in programming: Declaring and Using Objects. This lesson is for beginners to understand the basics of objects.
Summary
An object, as you probably already know, is the most raw type of class. It allows you to put together a unique collection of fields, properties, methods, etc. to be able to wrap common functionality into one single.. well.. object. It's practically the base class for most Delphi programming. Many people also tend to call a 'class' an 'object' quite often too.
More to be written in this FAQ to come!
Sample Code
We're going to create a new unit for this example. Within a Delphi project, go to File > New > Unit. Then copy this code below and replace whatever's in that nearly empty unit.
Don't let the double declaration of 'TMyObject' fool you. It's a good practice to first declare just the name of your object right under the 'type' clause, then further down continue its structure. This allows you to be able to refer to that object in any other object you may have declared before it. In other words, if you have two objects, and you want to declare each of them within each other, then you have to first pre-declare those classes just below `type`.
Of course the 'published' section doesn't do much good in this example, because it's only really meant for components to show their properties in the Object Inspector (where the component's properties are set). But it still works perfect if you want to put a clean object together.
Using Objects
Now you want to use this new TMyObject from your form's unit. Simply declare your new unit's name ('Unit2' in this example) in the uses clause at the very top of the form's unit. In Delphi, unit names are equivalent to a Namespace.
And right after you replace this code in your form, you need to immediately right-click the form, go to 'View as Text' and replace the 'DFM' code you see there with the code below. Then right-click the code and select 'View as Form'...
Don't try to modify anything in this DFM, unless you really know what you're doing.
When the application starts, it will first show the form, then it will pop up a message with the default parameter that was passed into the object when it was created. After this message, another one will appear saying that Three is not selected. Looking at the end of the code in the form create procedure will show you how to use a 'set'.
I will write more on this topic and add more data types in the future, when I have more spare time.
PS - Just for the record, I wrote most of the code above directly into this website, copied it over to my Delphi 7, and it compiled perfectly
Summary
An object, as you probably already know, is the most raw type of class. It allows you to put together a unique collection of fields, properties, methods, etc. to be able to wrap common functionality into one single.. well.. object. It's practically the base class for most Delphi programming. Many people also tend to call a 'class' an 'object' quite often too.
More to be written in this FAQ to come!
Sample Code
We're going to create a new unit for this example. Within a Delphi project, go to File > New > Unit. Then copy this code below and replace whatever's in that nearly empty unit.
Code:
unit Unit2;
interface
uses
Classes, Windows, SysUtils;
type
TMyObject = class;
TMyOtherObject = class;
TSomeType = (stOne, stTwo, stThree);
TSomeTypes = set of TSomeType;
TMyObject = class(TObject)
private
FSomeData: String;
FSomeTypes: TSomeTypes;
FMyOtherObject: TMyOtherObject;
procedure SetSomeData(const Value: String);
procedure SetSomeTypes(const Value: TSomeTypes);
protected
{ This is for another lesson }
public
constructor Create(SomeParam: String);
destructor Destroy; override;
property MyOtherObject: TMyOtherObject read FMyOtherObject;
published
property SomeData: String read FSomeData write SetSomeData;
property SomeTypes: TSomeTypes read FSomeTypes write SetSomeTypes;
end;
TMyOtherObject = class(TObject)
public
SomeString: String;
SomeInteger: Integer;
end;
implementation
constructor TMyObject.Create(SomeParam: String);
begin
//Create my other object
FMyOtherObject:= TMyOtherObject.Create;
//Set some param data
FSomeData:= SomeParam;
//Set some default values
FSomeTypes:= [stOne, stTwo, stThree];
end;
destructor TMyObject.Destroy;
begin
//Free my other object
FMyOtherObject.Free;
end;
procedure TMyObject.SetSomeData(const Value: String);
begin
FSomeData:= Value;
end;
procedure TMyObject.SetSomeTypes(const Value: TSomeTypes);
begin
FSomeTypes:= Value;
end;
end.Don't let the double declaration of 'TMyObject' fool you. It's a good practice to first declare just the name of your object right under the 'type' clause, then further down continue its structure. This allows you to be able to refer to that object in any other object you may have declared before it. In other words, if you have two objects, and you want to declare each of them within each other, then you have to first pre-declare those classes just below `type`.
Of course the 'published' section doesn't do much good in this example, because it's only really meant for components to show their properties in the Object Inspector (where the component's properties are set). But it still works perfect if you want to put a clean object together.
Using Objects
Now you want to use this new TMyObject from your form's unit. Simply declare your new unit's name ('Unit2' in this example) in the uses clause at the very top of the form's unit. In Delphi, unit names are equivalent to a Namespace.
Code:
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, Unit2, StdCtrls;
type
TForm1 = class(TForm)
Edit1: TEdit;
Button1: TButton;
procedure FormCreate(Sender: TObject);
procedure FormDestroy(Sender: TObject);
procedure Button1Click(Sender: TObject);
private
//Declare object in form's class if it goes hand-in-hand with the form
FMyObject: TMyObject;
public
property MyObject: TMyObject read FMyObject;
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
var
O: TMyOtherObject;
begin
//Next 3 lines are just a trick to show the form before anything is done...
Show;
BringToFront;
Application.ProcessMessages;
//Temporarily create/free an object to save memory...
O:= TMyOtherObject.Create;
try
//Do something with 'O'
O.SomeString:= 'This is some string';
O.SomeInteger:= 2011;
finally
O.Free;
end;
//Create our new object...
FMyObject:= TMyObject.Create('This is some sample parameter');
//Set some default values...
FMyObject.SomeTypes:= FMyObject.SomeTypes - [stThree];
//Get something from our new object
ShowMessage(MyObject.SomeData);
Edit1.Text:= MyObject.SomeData;
if stThree in MyObject.SomeTypes then begin
ShowMessage('Three is selected!');
end else begin
ShowMessage('Three is NOT selected!');
end;
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
//Now we have to free our new object
FMyObject.Free;
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
Close;
end;
end.And right after you replace this code in your form, you need to immediately right-click the form, go to 'View as Text' and replace the 'DFM' code you see there with the code below. Then right-click the code and select 'View as Form'...
Code:
object Form1: TForm1
Left = 444
Top = 444
Width = 313
Height = 221
Caption = 'My New Object Sample'
Color = clBtnFace
Font.Charset = DEFAULT_CHARSET
Font.Color = clWindowText
Font.Height = -11
Font.Name = 'MS Sans Serif'
Font.Style = []
OldCreateOrder = False
Position = poScreenCenter
OnCreate = FormCreate
OnDestroy = FormDestroy
DesignSize = (
297
183)
PixelsPerInch = 96
TextHeight = 13
object Edit1: TEdit
Left = 24
Top = 40
Width = 249
Height = 21
Anchors = [akLeft, akTop, akRight]
TabOrder = 0
end
object Button1: TButton
Left = 80
Top = 88
Width = 121
Height = 33
Anchors = [akLeft, akTop, akRight]
Caption = 'Close'
Font.Charset = DEFAULT_CHARSET
Font.Color = clWindowText
Font.Height = -11
Font.Name = 'MS Sans Serif'
Font.Style = [fsBold]
ParentFont = False
TabOrder = 1
OnClick = Button1Click
end
endDon't try to modify anything in this DFM, unless you really know what you're doing.
When the application starts, it will first show the form, then it will pop up a message with the default parameter that was passed into the object when it was created. After this message, another one will appear saying that Three is not selected. Looking at the end of the code in the form create procedure will show you how to use a 'set'.
I will write more on this topic and add more data types in the future, when I have more spare time.
PS - Just for the record, I wrote most of the code above directly into this website, copied it over to my Delphi 7, and it compiled perfectly